Reservoir Water Quality Analysis

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CECW-EH

Engineer

Manual

1110-2-1201

Department of the Army

U.S. Army Corps of EngineersWashington, DC 20314-1000

EM 1110-2-

1201

30 Jun 1987

Engineering and Design

RESERVOIR WATER QUALITY ANALYSIS

Distribution Restriction Statement

Approved for public release; distribution is unlimited.


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CEEC-EHCECW-PCECW-O

DEPARTMENT OF THE ARMY EM 1110-2-1201U. S. Army Corps of Engineers

Washington, DC 20314-100030 June 1987

Engineer ManualNo. 1110-2-1201

Engineering and DesignRESERVOIR WATER QUALITY ANALYSIS1. Purpose. This manual provides guidance for the assessmentof reservoir water quality conditions, including reservoir pool,releases and tailwaters.2. App liability. This manual applies to all HQUSACE/OCEelements and field operating activities (FOA) having responsi-bility for water quality/quantity control. It provides aframework to guide Corps of Engineers scientists and engineers inassessing water quality conditions associated with reservoirs.3. Discussion. Early reservoir water quality assessment acti-vities were based on techniques and processes commonly acceptedin standard limnology and sanitary engineering. However,approaches to assessing and solving reservoir water qualityproblems were often found to be insufficient due to processes andwater control operations inherent to reservoirs. Some of theseproblems interfere with project purposes and/or result directlyfrom water control (reservoir regulation) practices. Many ofthese problems were addressed by the Environmental and WaterQuality Operational Studies (EWQOS) research program, and newtechnologies pertinent to reservoir water quality have beendeveloped. Much of the material in this manual is a product ofthis program and of field experience from Corps district anddivision offices.FOR THE COMMANDER:

qk cR bert . Lee Colonel, CorpsChief of Staff

of Engineers


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CEEC-~HEngineer ManualNo. 1110-2-1201

DEPARTMENT OF THE ARMYU. S. Army Corps of EngineersWashington, DC 20314-1000

Engineering and DesignRESERVOIR WATER QUALITY ANALYSIS

Table of Contents

SubjectCHAPTER 1Section I.

Section II.

CHAPTER 2Section I.

Section II.

Section III.

Section IV.

CHAPTER 3Section I.

INTRODUCTIONGeneralPurpose. . . . . . . . . . . . . . . . . . .Applicability. . . . . . . . . . . . . . . .References . . . . . . . . . . . . . . . . .Background . . . . . . . . . . . . . . . . .Water Quality Assessment in WaterQuality Control ManagementGeneral. . . . . . . . .Planning and Analysis . .Water Control Management

WATER QUALITY PARAMETERSIntroduction

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Definition of Water Quality . . . . . . .Reservoir-WatershedRelationship . . . .Reservoir DescriptionDefinition . . . . . . . . . . . . . . .Comparison to Natural Lakes . . . . . . .Classification of Reservoirs . . . . . .Reservoir Characteristics and ProcessesGeneral. . . . . . . . . . . . . . . . .Physical Characteristics and ProcessesChemical Characteristics of ReservoirProcesses. . . . . . . . . . . . . . .Biological Characteristics and ProcessesReleases and TailgatersReleases . . . . . . . . . . . . . . . .Tailwaters . . . . . . . . . . . . . . .Characteristics and Processes . . . . . .WATER QUALITY ASSESSMENTDesigning the Assessment PlanEstablishing Objectives . . . . . . . . .

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Subject Paragraph PageDesign Considerations . . . . . . . . . . . . 3-2 3-11Elements of Assessment . . . . . . . . . . . 3-3 3-12

Section II. Water Quality Assessment Program/Study CategoriesPref.mpoundmentAssessment . . . . . . . . . .

Postimpoundment Assessment . . . . . . . . .Operational Monitoring . . . . . . . . . . .Modification of Operations . . . . . . . . .Modification of Water ControlStructures . . . . . . . . . . . . . . . .Specific Water Quality Problems . . . . . . .

CHAPTER 4 WATER QUALITY ASSESSMENT TECHNIQUESScope. . . . . . . . . . . . . . . . . . . .Section 1. Screening TechniquesGeneral. . . . . . . . . . . . . . . . . . .

Information Search . . . . . . . . . . . . .Project Characteristics andCalculations . . . . . . . . . . . . . . .Site-Specific Water Quality Data . . . . . .Section II. Diagnostic TechniquesGeneral. . . . . . . . . . . . . . . . . . .Field Investigations . . . . . . . . . . . .Laboratory Studies . . . . . . . . . . . . .Statistical Techniques . . . . . . . . . . .Water Quality Indices . . . . . . . . . . . .Remote Sensing . . . . . . . . . . . . . . .Section III. Predictive TechniquesGeneral. . . . . . . . . . . . . . . . . . .

Regression Analysis . . . . . . . . . . . . .Comparative Analysis . . . . . . . . . . . .Modeling . . . . . . . . . . . . . . . . . .Nutrient Loading Models . . . . . . . . . . .Numerical Simulation Models . . . . . . . . .Physical Models . . . . . . . . . . . . . . .CHAPTER 5 WATER QUALITY DATA COLLECTION AND ANALYSISSection I. IntroductionPurpose. . . . . . . . . . . . . . . . . . .Overview . . . . . . . . . . . . . . . . . .Section II. Field Data CollectionPrinciples . . . . . . . . . . . . . . . . .

Sampling Designs . . . . . . . . . . . . . .Field Sampling and Analysis . . . . . . . . .Laboratory Analysis . . . . . . . . . . . . .

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SubjectSection III. Database ManagementDatabase Management Systems . . . . . . . . .Selection Criteria . . . . . . . . . . . . .

Section IV. Data Presentation

APPENDIX A

APPENDIX BAPPENDIX C

APPENDIX D

Methods. . . . . . . , . . . . .Summary Tables . . . . . . . . .Graphic Displays . . . . . . . .Quality Assurance . . . . . . . .Statistical Analysis . . . . . .REFERENCESDepartment of the AruIy,Corps ofEngineers. . . . . . . . . . .Department of the Army, Corps ofEngineers, Waterways ExperimentStation. . . . . . . . . . . .

Other Government Publications . .Nongovernment Publications . . .BIBLIOGWPHYSAMPLE DESIGN CALCULATIONS:DEVELOPMENT OF DECISION MATRIX

(TABLE 5-I)ORDER OF MAGNITUDE ESTIMATESPurpose. . . . . . . . . . . . .Morphometric and Hydrologic

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Characteristics . . . . . . . . . . .Physical Relationships . . . . . . . .Chemical Relationships . . . . . . . .

GLOSSARY

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List of Tables2-1 Selected Trophic Indicators and Their Response toIncreased Eutrophication . . . . . . . . . . . . . . .2-2 Physical, Chemical, Morphometric, and HydrologicRelationships. . . . . . . . . . . . . . . . . . . . .2-3 Nutrient Demand:Supply Ratios During Nonproductiveand Productive Seasons . . . . . . . . . . . . . . . .

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List of Tablee3-1 Water Quality Concerns and Possible ContributingFactors. . . . . . . . . . . . . . . . . . . . . . . .4-I Summary of Existing Instream Flow Assessment

Methods . . . . . . . . . . . . . . . . . . . . . . . .4-2 Summary of Simulation Model Attributes . . . . . . . . .5-1 Example of a Sampling Matrix to Optimize SampleNumbers, Precision, and Cost . . . . . . . . . . . . .5-2 Example of Sampling Intervals Corresponding withHydrologic and Ltiological Periods . . . . . . . . . .5-3 Typical Water Quality Variables Measured inReservoirs and the Sample Handling and Preser-vation Requirements . . . . . . . . . . . . . . . . . .5-4 Example of Description Statistics That Can BeApplied to Water Quality Data . . . . . . . . . . . . .5:5 Summary of Parametric Statistical Tests . . . . . . . . .5-6 Summary of Nonparametric Statistical Tests . . . . . . .

List of Figures2-1 Longitudinal patterns in reservoir waterquality. . . . . . . . . . . . . . . . . . . . . . . .2-2 Vertical zonation resulting from thermalstratification . . . . . . . . . . . . . . . . . . . .2-3 Illustration of bottom withdrawal structure,spillway, and stilling basin . . . . . . . . . . . . .2-4 Example of a surface withdrawal structure . . . . . . . .2-5 Dual wet well multilevel withdrawal structure . . . . . .2-6 Water density as a function of temperature . . . . . . .2-7 Recurring annual stratification pattern fortemperate reservoir . . . . . . . . . . . . . . . . . .2-8 Density inflows to reservoirs . . . . . . . . . . . . .

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List of Fi~ures2-92-1o

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Internal mixing processes in reservoirs . . . . . . . . .Influence of penetrative convective mixing ondeepening the mixing layer . . . . . . . . . . . . . .

Important hydrodynamic features of pumped-storagereservoirs subject to jetting inflows . . . . . . . . .

Characteristic metalimnetic DO minimum . . . . . . . . .Orthograde and clinograde vertical DO distribution 2-28Seasonal phosphorus flux under aerobic and anaero-bicconditions . . . . . . . . . . . . . . . , . . . .

Seasonal patterns of phytoplankton succession . . . . . .Lateral distribution of macrophytes in littoralzone . . . . . . . . . . . . . . . . . . . . . . . . .Generalized reservoir ecosystem indicating physical,chemical, and biological interactions includinghigher trophic levels . . . . . . . . . . . . . . . .Potential effects and interactions of modified flow .regime on downstream biota . . . . . . . . . . . . . .

Schematic of a hydropower facility . . . . . . . . . . .POC concentrations and transport during hydropowergeneration cycle . . . . . . . . . . . . . . . . . . .

Contributing factors and potential consequences ofalgae blooms . . . . . . . . . . . . . . . . . . . . .Contributing factors and potential consequences ofaquatic weed infestations . . . . . . . . . . . . . . .Contributing factors and potential consequences ofhigh bacterial counts . . . . . . . . . . . . . . . . .Contributing factors and potential consequences ofaltered release temperatures . . . . . . . . . . . . .Contributing factors and potential consequences ofrelease DO concentrations . . . . . . . . . . . . . . .

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List of Figures3-6

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Contributing factors and potential consequences ofgas supersaturation . . . . . . . . . . . . . . .Contributing factors and potential consequences offish stress on both the reservoir and tailwaterfishery. . . . . . . . . . . . . . . . . . . . .

Project characteristics known prior to beginning awaterqualfty study . . . . . . . . . . . . . . .Typical water quality factors to be assessed inreservoir water quality studies . . . . . . . . .

Selected information retrieval services . . . . . .Control pathways in a typical nutrient loadingmodel. . . . . . . . . . . . . . . . . . . . . .

Comparison of model dimensions . . . . . . . . . .Typical water quality model constituents andpathways . . . . . . . . . . . . . . . . . . . .Model representation for conservation of mass . . .Statistical comparison of computer simulationresults for two management alternatives . . . . .

Potential bias in sampling program using a fixedinterval (e.g., 30-day) sampling period . . . . .

Exampledataplots . . . . . . . . . . . . . . . .Comparison of graphic display methods . . . . . . .Example scatter diagrams . . . . . . . . . . . . .

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CHAPTER 1INTRODUCTION

Section I. General1-1. Purpose. This manual provides guidance for the assessment of reservoirwater quality conditions, including reservoir releases and tailgaters.Procedures are generally presented without theoretical discussion, since thesedetails can be found in referenced sources.1-2. Applicability. This manual applies to all field operating activitieshaving responsibilities for reservoir water quality/ quantity control. Itprovides a framework to guide Corps of Engineers scientists and engineers Inassessing water quality conditions associated with reservoirs. Emphasis isplaced on procedures to define program and/or study objectives and to selectappropriate techniques for assessing water quality conditions in the planning,design, and water control management of reservoirs.1-3. Reference. The references are indicated throughout the manual by numbersthat correspond to similarly numbered items in Appendix A.1-4. Background.

a. Environmental concern expressed by the public through the Congresshas resulted in the passage of Federal legislation and the issuance ofExecutive Orders directing increased efforts by Federal agencies in waterquality management. Initial legislation on water quality management wasdirected toward public health and water supply. Subsequent legislation andExecutive Orders, such as the Federal Water Pollution Control Act Amendments of1977 (PL 95-217, 33 U.S.C 1323 et seq., the Clean Water Actn), and ExecutiveOrder 12088 (Federal Compliance with Pollution Control Standards,W 13 October1978), placed the responsibility for compliance with local and state wllutionabatement laws with directors of Federal agencies. Corps policies andauthorities relative to water quality are contained in ER 1130-2-334, ER 1105-2-50, and EP 1165-2-1.

b. Early reservoir water quality assessment activities were based ontechniques and referenced processes commonly accepted in standard limnology andsanitary engineering. However, previous approaches to assessing and solvingreservoir water quality problems were sometimes found to be lacking due toprocesses and water control operations inherent to reservoirs. Some of thesewater quality problems interfere with project purposes and/or result directlyfrom water control (reservoir regulation) practices. Many of these problemswere addressed by the Environmental and Water Quality Operational Studies(EWQOS) research program, and new technologies pertinent to reservoir waterquality have been developed. Wch of the material in this manual is a product

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of this program and of field experience from Corps district and divisionoffices.Section II. Water Quality Assessment inWater Quality Control Management

1-50 General. Water quality assessments of reservoirs are designed and con-ducted to meet specific reservoir use objectives. These assessments areintended either for predicting future conditions, such as the reservoir waterquality in a proposed impoundment or the tailwater quality resulting from pro-posed changes in the water control plan at an existing project, or fordescribing existing conditions, such as postimpoundment quality. In addition,the results of water quality assessments serve as source material for environ-mental impact statements and assessments, project water control manuals,recreation master plans, and future projects.1-6. Planning and Analysis. The stage of the reservoir project investigationdetermines the extent of resources available and, therefore, the depth of awater quality assessment. Obviously, a reservoir water quality assessmentmade during the early stages of a project reconnaissance investigation is gen-erally less intensive and definitive than assessments conducted during feasi-bility studies or those made for project feature design and environmentalimpact determination in the post-authorizationphase.

a. Reconnaissance Studies.(1) During the early phases of project planning investigations (recon-naissance), it is important to make an initial information search (Chapter 4,para 4-3) and determine existing water quality conditions in the watershedunder study. Factors such as elevated levels of certain water quality consti-tuents, municipal and industrial point-sources of pollution, land use prac-

tices, municipal water supply requirements, State stream water quality stan-dards, and other water uses should be identified. These factors are extremelyimportant in determining water quality assessment requirements and objectivesfor the next phase of the planning investigation.(2) Limited resources and the fact that specific reservoir sitings and

project purposes are not yet fully developed during the reconnaissance phaseusually preclude the need for extensive field data collection or use of thediagnostic and predictive techniques described in Chapter 4.(3) When the planning investigation progresses to the point at whichalternative reservoir sites are considered, the process of assessing futurereservoir water quallty conditions begins. Usually, resources at this stage

permit only limited field data collection. Predictive techniqueswill alsoordinarily be restricted to the use of regression type and/or comparative typeanalyses (Chapter 4). The requirements at the stage of study are to provide ageneral indication of the proposed impoundment in terms of whether it will bestrongly stratified, will have low dissolved oxygen or other gas

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concentrations, and related water quality problems that would adversely affectproject purposes or require special water control features (e.g.,multilevelwithdrawal structure or reaeration facility) for mitigation and control. Thisinformation will be used to scope the level and extent of the water qualityassessment needed for the feasibility investigation.

b. Feasibility Studies.(1) During the feasibility investigation (development of the recommendedplan and preparation of the environmental impact statement), it may be neces-

sary to use nutrient loading, thermal simulations, and/or comprehensive waterquality models to predict future water quality and determine the need for spe-cific water quality control features. In most cases, use of rigorous simula-tion models will require more water quality data than those gathered duringthe earlier planning work, and additional data collection and analysis will berequired.(2) Another analysis is to determine whether the proposed impoundment

lands, through the presence of vegetation and certain soil types, will con-tribute to water quality degradation (see Chapter 4). Sufficient resources toconduct these water quality studies should be provided in the feasibilityinvestigation. Also, the water quality control features and their associatedoperational requirements must be considered in the investigation so that accu-rate estimates of project costs and benefits can be made.

c. Post-authorization Studies. During the post-authorizationphase,detailed design of project features and definitive environmental impact deter-minations are prepared. Resources must be programmed to conduct data collec-tion, while water quality simulation studies may be conducted to select thetype and determine the specific geometry of water control structures (e.g.,gates, submerged weirs, stilling basins) to meet project purposes and waterquality objectives. The studies during this stage will usually encompass useof the more rigorous water quality simulation models (Chapter 4). They may,in addition, require physical models to define project-specifichydrodynamicconditions in the reservoir and tailwater for subsequent use in mathematicalmodels and/or for direct application to design. Guidance on hydrologic inves-tigation requirements for water quality control is contained inER 1110-2-1402.I-7. Water Control Management.

a. Water quality assessments of existing reservoirs can vary in com-pleteness and detail depending on the objective of the assessment. For com-pliance with ER 1130-2-334, it may be sufficient to carry out a monitoringprogram without resorting to modeling (especiallywhen no water quality prob-lems are identified at the resenoir). In this instance, trend-monitoringtoidentify possible problems/conditionsmay be the activity required.

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b. For a reservoir in which a specific water quality condition has beenidentified or to which a change in project use is proposed (e.g., hydropowerretrofit), the assessment may be as extensive as that used in post-authori-zation design studies. In this case, there may be need for extensive fieldsampling and laboratory analysis (Chapter 5) and for evaluation techniques suchas mathematical and/or physical modeling. Further, correcting a water qualityproblem or meeting requirements of the new project purpose may require thedesign and construction of modifications to the outlet works and/or modi-fications to the water control plan.

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CHAPTER 2WATER QUALITY PARAMETERSSection I. Introduction

2-1. Definition of Water Quality. Water quality, as defined in this manual,is composed of the physical, chemical, and biological characteristics of waterand the abiotic and biotic interrelationships.2-2. Reservoir-WatershedRelationship.

a. Any reservoir or stream system is coupled with its watershed ordrainage basin. Therefore, basin geometry, geology, climate, location, andland use are integral factors that directly or indirectly influence stream orreservoir water quality. Conversely, water quality changes in reservoirs arethe result of physical, chemical, and biological loading, generally throughrunoff and/or stream transport and processing.

b. In a dam/reservoir project area, the Corps owns a limited quantity ofthe surrounding land. As a result, the water quality of a particular reser-voir is often controlled by a watershed, and/or activities therein, over whichthe Corps has little or no control. In turn, many water quality problems inthe reservoir cannot be dealt with directly but must be handled by or througha local, state, or other Federal entity. However, one should not assume thatall water quality problems are the result of the watershed characteristicsalone. Many water quality problems result from structures associated with thedam, project operation, or the reservoir itself. Solutions to these problemsare within the control of the Corps.

Section 11. Reservoir Description2-3. Definition. In Iimnological terminology (study of freshwater bodies),reservoirs are defined as artificial lakes. All standing waters were classi-fied as lakes as far back as the 1890s by the pioneer limnologist Forel.More recently, lakes have been classified into 76 types, with reservoirs asone type of lake produced by higher organisms, that is, man (see Ref. 77).2-4. Comparison to Natural Lakes. In some ways, reservoirs can be consideredas having the characteristics of only one-half of natural lakes. That is, thedeepest portion of a natural lake may be located anywhere, but is often nearthe center, with all portions of the lake bottom sloping toward that maximumdepth. By contrast, the deepest portions of reservoirs are almost always nearthe dam, and the reservoir bottom usually slopes toward the dam. Also, theinlet and outlet of natural lakes are near the surface, whereas a reservoircan release water from any location, ranging from the surface to the deepestportion of the impoundment. Consequently, although the limnological processesdetermining water quality conditions are the same in both cases, the hydro-dynamics of reservoirs make their water quality characteristics different than

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those of natural lakes. From an ecological point of view, a reservoir nor-mally has variable productivity potential levels--high in the early years, lowduring the following years and then, sometimes, high again during the reser-voirs mature stage. By contrast, the natural lake follows a successionalpattern from oligotrophy to eutrophy.2-5. Classification of Reservoirs. Reservoirs, especially natural lakes,have been classified using a variety of systems, including physical, chemical,and geomorphologicalcharacteristics,and indicator species or species aggre-gates. This section presents a brief overview of the classification systemscommonly used within the Corps.

a. Stratified Versus Unstratified. Reservoirs may or may not stratify,depending on conditions such as depth, wind mixing, and retention time (seepara 2-7d). Under appropriate conditions, the reservoir will form an epi-limnion or upper layer, a metalimnion or transitional layer, and a hypolimnionor lower layer. However, if conditions do not allow stratification, the en-tire reservoir may consist of an epilimnion with an isothermal gradient. Thestratified or unstratified condition can dramatically affect water qualityconditions of the reservoir and its releases. Releases from an unstratifiedreservoir, irrespective of the withdrawal level, will generally be warmwaterreleases; bottom-level withdrawals from a stratified reservoir will be gen-erally coldwater releases. Warm and cold releases, in the sense of this dis-cussion, are relative to the water temperature of the stream into which thereleases are made. Additional aspects of water quality conditions associatedwith stratified or unstratified conditions will be discussed in subsequentsections.

b. Operational Characteristics.(1) General. Reservoir projects are authorized for a variety of pur-

poses, the most common of which are flood control, navigation, hydroelectricpower generation, water supply, fish and wildlife conservation and enhance-ment, recreation, and low-flow augmentation. Since the mid-1970s, Corps res-ervoirs also have water quality enhancement as an authorized project purpose.Today, most reservoirs are authorized as multiple-purpose projects, with stor-age allocated for two or more purposes. Multiple-purpose reservoirs, operatedeither separately or as a system, often result in conflicting uses for reser-voir storage.

(2) Flood control. Use of a reservoir for flood control consists ofstoring water in excess of the downstream channel capacity (damaging flows)during flood periods for later release during periods of flow at or belowchannel capacity (nondamaging flows) at a downstream control point. Since amajor factor in flood control reservoirs is maintaining available volume(i.e., empty storage space) for flood storage, the flood control purposegenerally is the least compatible with other project purposes.

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(3) Navigation. Reservoir projects operated for navigation purposes aredirected at providing sufficient downstream flow to maintain adequate waterdepth for navigation and/or providing sufficient water volume for lockages.In many navigation projects, the reservoir pool is a part of the channel, sopool levels must be controlled to provide both sufficient navigation depthswithin the pool and downstream depths. Downstream releases for navigationpurposes may have a distinct seasonal pattern, with higher releases requiredduring the dry season.

(4) Hydroelectric power generation.(a) Hydroelectric power generation consists of passing water throughturbines to produce electricity. Hydroelectric facilities normally are oper-ated to produce two types of power: baseload power and peak power. Baseloadpower is firm power generated to supply a portion of a constant daily demandfor electricity. Peaking power is power supplied above the baseload to sat-

isfy variable demands during periods of heavy electricity usage. Reservoirreleases to meet baseload power are generally constant over several hours,while those designed for peaking power will fluctuate by the hour.

(b) The power output of a facility is determined by the flow through theturbine and the head or pressures exerted on the turbine. Therefore, it isadvantageous to have hydroelectric power reservoirs at maximum storage or fullpool for maximum power generation. Thfs is best accomplished by pumped-storage hydropower reservoirs which maintain a full pool by pumping previouslyreleased water back into the reservoir following generation. Electricity Isgenerated and used during high-demand periods (i.e., high market value) toprovide energy to consumers; it is used to pump water back into the reservoirduring low-demand periods when energy costs are lower. Most hydroelectricpower plants are part of either an interconnected system or a power grid sothat flexibility in coordinating generation releases with other water uses ispossible.

(5) Water supply. Reservoirs with water supply objectives store waterduring periods of excess inflow for use during other periods. Withdrawal maytake place directly from the reservoir, or in downstream reservoir releases.Water is generally provided to municipal, industrial, or agricultural users asreservoir storage rather than by contract to supply a specific volume ofwater. Consequently, water supply can be obtained by a user from the reser-voir as long as there is sufficient water in that particular segment of stor-age. Adequate reserve storage is usually maintained to avoid water shortagesduring drought periods.

(6) Fish and wildlife conservation and enhancement. Reservoirs used forfish and wildlife conservation and enhancement may include features such asintake structures to minimize entrapment and entrainment of fish and otheraquatic species; outlet and emergency spillway structures to minimize contactof aquatic species with waters supersaturatedwith dissolved gases and to pro-vide appropriate release water quality; and fish ladders, fish bypasses, an>

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other pertinent facilities to permit fish passage around structures. Fish andwildlife habitat at these reservoirs is improved by retaining standing vegeta-tion during construction, as well as providing conditions conducive to growthof suitable aquatic and wetland vegetation.

(7) Recreation. Recreation activities in and around reservoir projectsinclude camping, picnicking, fishing, pleasure boating, water skiing, swim-ming, and hunting. Similar activities also take place downstream of the res-ervoir in and adjacent to the tailwater. Recreational users of both areasgenerally prefer constant water levels.

(8) Low-flow augmentation. Low-flow augmentation reservoirs providereleases that increase flow in the downstream channel for downstream fish andwildlife purposes or for downstream water quality control. Storage allocationfor downstream water quality control currently can be obtained only under spe-cial circumstances.

c. Trophic Status.(1) Reservoirs are commonly classified or grouped by trophic or nutrient

status. The natural progression of water bodies through time is from anoligotrpphic (i.e., low nutrient/low productivity) through a mesotrophic(i.e., intermediate nutrient/intermediate productivity) to a eutrophic (i.e.,high nutrient/high productivity) condition. The prefixes ultra and hyperare sometimes added to oligotrophic and eutrophic, respectively, as additionaldegrees of trophic status. The tendency toward the eutrophic or nutrient-richstatus is common to all impounded waters.

(2) The eutrophication or enrichment process has received considerablestudy because:(a) It can be accelerated by nutrient additions through cultural activ-ities (e.g., point-source discharges and nonpoint sources such as agriculture,urbanization, etc.).(b) Water quality conditions associated with eutrophication may not bedesired.(c) To a certain degree, cultural eutrophication impacts are reversible.(3) The majority of reservoir water quality conditions relate to theeutrophication process. Certain physical, chemical, and biological factors

change during eutrophication (Table 2-1). Quantitative criteria for thesefactors have been developed to define various trophic states, but the rangesare broad and may not reflect geographic/demographicdifferences in waterquality. (Additional discussion of eutrophication can be found in Refs. 43,44, 45, and 110 and in Item ff of Appendix B.)

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TABLE 2-1Selected Trophic Indicators and Their Response to

Increased Eutrophication

Physical Chemical BiologicalTransparency (D) Nutrient concentrations (I) Algal bloom frequency (1)(Secchi disk depth) (e.g., at spring maximum) Algal speciesSuspended solids (I) Chlorophyll ~ (I) diversity (D)

Conductivity (I) Littoral vegetation (1)Dissolved solids (I) Zooplankton (1)Hypolimnetic oxygen Fish (I)deficit (I) Bottom fauna (I)

Epilimnetic oxygensupersaturation (I) Bottom faunadiversity (D)

Primary production (I)Phytoplankton biomass (I)

1(1) = Increased, (D) = decreased.2-5


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Section III. Reservoir Characteristics and Processes2-6. General.

a. Reservoir water quality is a system response to the reservoirswatershed, the regions climate, as well as the geometry and internal charac-teristics and processes of the reservoir. Water quality is affected by thetype, location, and manner of operation of the reservoirs water controlfacilities. Macro- and micro-meteorological forces, inflows, internal pro-cesses, outflows, and project operation are highly dynamic and can be dominantfactors in determining the water quality in a reservoir. To understand whycertain water quality conditions develop, one must understand the interactionof all the dynamic phenomena influencing the reservoir and its associatedwaters.

b. This section introduces some of the important characteristics andprocesses that influence the quality of water in reservoirs. For simplicity,relevant limnological factors are categorized as being physical, chemical, orbiological in nature. Such separation does not occur in nature; the factorsare all interrelated. Thus, it must be understood that many factors discussedcould fall into more than one category. (Additional information on limnologi-cal processes and terminology can be found in Refs. 77, 78, and 110.)2-7. Physical Characteristics and Processes.

a. Site Preparation. Depending upon the planned reservoir uses, sitepreparation (e.g., topsoil stripping, timber removal) may have a significanteffect upon water quality after inundation. Additional information on thesubject may be found in Refs. 13 and 71.b. Morphometry.(1) Morphometric variables that can influence hydrologic and limnologiccharacteristicsof the reservoir include surface area, volume, mean depth,maximum depth, shoreline development ratio, and fetch. Formulas for computing

the values of these and other characteristics are given in Table 2-2. Biolog-ical productivity, respiration, decomposition, and other processes influencingwater quality are related directly or indirectly to reservoir morphometry.Morphometric characteristics themselves also are interrelated and provideinsight into existing or potential water quality conditions. Mean depth, forexample, is computed as volume/surface area (V/A); shallow mean depths mayindicate light penetration to the bottom, warmer water temperatures,higherorganic decomposition rates, and greater nutrient regeneration. All thesefactors can contribute to higher productivity. Lakes with shallow mean depthsgenerally have higher biological productivity than lakes with deeper meandepths with comparable surface areas.

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(2) Reservoirs with high shoreline development ratios are indicative ofdendritic systems with many coves and embayments, while low values of theratio are often indicative of more prismatic type reservoirs. Biological pro-ductivity usually is higher in coves than in the main pool; thus, reservoirshaving high shoreline development ratios tend to be more productive.

3) Fetch is the distance over water that the wind has blown uninter-rupted by land. When computed along the direction of the prevailing wind, thefetch length can provide an indication of wave heights and potential erosionareas on the windward reservoir side where the waves will break.4) The shape of area-capacity curves integrates morphometric parametersthat relate to biological productivity. Reservoirs with flatter slopes on thearea-elevation, elevation-volume curves usually have higher productivity.c. Longitudinal Gradients. Reservoirs can exhibit pronounced longitu-dinal and vertical physical, chemical, and biological gradients. Long, den-dritic reservoirs, with tributary inflows located a considerable distance from

the outflow and unidirectional flow from headwater to dam, develop gradientsin space and time. Although these gradients are continuous from headwater todam, three characteristic zones result: a riverine zone, a zone of transi-tion, and a lacustrine zone (Figure 2-1).

(1) Riverine zone. The riverine zone is relatively narrow, well mixed,and although water current velocities are decreasing, advective forces arestill sufficient to transport significant quantities of suspended particles,such as silts, clays, and organic particulate. Light penetration in thiszone is minimal and may be the limiting factor that controls primary produc-tivity in the water column. The decomposition of tributary organic loadingsoften creates a significant oxygen demand, but an aerobic environment is main-tained because the riverine zone is generally shallow and well mixed. Longi-tudinal dispersion may be an important process in this zone.

(2) Zone of transition. Significant sedimentation occurs through thetransition zone, with a subsequent increase in light penetration. Light pene-tration may increase gradually or abruptly, depending on the flow regime. Atsome point within the mixed layer of the zone of transition, a compensationpoint between the production and decomposition of organic matter should bereached. Beyond this point, production of organic matter within the reservoirmixed layer should begin to dominate (Figure 2-1).

3) Lacustrine zone. The lacustrine zone is characteristic of a lakesystem (Figure 2-1). Sedimentation of inorganic particulate is low; lightpenetration is sufficient to promote primary production, with nutrient levelsthe limiting factor; and production of organic matter exceeds decompositionwithin the mixed layer. Entrainment of metalimnetic and hypolimnetic water,particulate, and nutrients may occur through internal waves or wind mixingduring the passage of large weather fronts. Hypolimnetic mixing may be moreextensive in reservoirs than lakes because of bottom withdrawal. Bottom

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INFLOW I I I III

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Figure 2-1. Longitudinal patterns in reservoir waterquality (after Item y, Appendix B)

withdrawal removes hypolimnetic water and nutrients and may promote movementof interflows or underflow into the hypolimnion. In addition, an intakestructure may simultaneouslyremove water from the hypolimnion andmetalimnion.d. Vertical Gradients. Attaining reservoir water quality objectives can

be significantly affected by vertical stratification in the reservoir. Thisstratification typically occurs through the interaction of wind and solar iso-lation at the reservoir surface and creates density gradients that can influ-ence reservoir water quality (see Figure 2-2). Stratification also can resultfrom density inflows (see para 2-7i) or high total dissolved solids (TDS) or

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TEMPERATURE

EPILIMNION

METALIMNION

HYPOLIMNION

Figure 2-2. Vertical zonation resulting fromthermal stratificationsuspended solids (SS) concentrations. Because of density stratificationandits sensitivity to meteorological conditions and tributary inflows, properhydraulic outlet design is imperative to ensure that reservoir and releasewater quality objectives can be met. Reservoir hydraulic outlet designsinclude the capability for bottom, surface, and multilevel withdrawal; low-flow releases; or the passing of large flows over a spillway.

(1) Bottom withdrawal. Bottom withdrawal structures are located nearthe deepest part of a reservoir (Figure 2-3). Historically, bottom withdrawalstructures have been the most common outlet structures used to release reser-voir waters. They release cold waters from the deep portion of the reservoir;however, these waters may be anoxic during periods of stratification. Bottomoutlets can release density interflows or underflows (e.g., flow laden withsediment or dissolved solids) through the reservoir and generally providelittle or no direct control over release water quality. In order to controlrelease water quality in projects with bottom outlets, external techniquessuch as release aeration, hypolimnetic aeration, or localized mixing must beused.

(2)from near Surface withdrawal. Surface withdrawal structures release watersthe surface of the reservoir pool (Figure 2-4) and include morning

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Figure 2-4. Example of surface withdrawal structureglory, drop inlet siphon, or shaft inlets. Outlet structures at the surfacegenerally release relatively warm, well-oxygenated waters. Surface outletsmust be designed to operate within the range of fluctuation of the reservoirwater surface. The surface outlet becomes unusable once the reservoir watersurface elevation falls below the crest of a surface outlet structure. Den-sity interflows or underflow cannot be released using surface outlets, nor isthere any direct control over the water quality of the release usingthe out-let structure. Few external techniques for controlling water quality withinthe reservoir can be used to control the water quality of releases from sur-face structures.

3) Multilevel withdrawal. Multilevel withdrawal structures have one ormore outlet towers, each containing a number of inlet ports at different ele-vations (Figure 2-5). This configuration provides the flexibility to releasewater from several levels within the reservoir. Designing port locations atvarious elevations may permit reservoir operation to meet release water qual-ity objectives by withdrawing water with the desired quality from appropriateelevations in the reservoir. However, in a single tower, only one port can beeffectively operated at any time if the reservoir is stratified. Operatingtwo ports at different elevations simultaneously in a stratified reservoirwith a single wet well can result in density blockage of flow, flow instabil-ity, pulsating release quality, and overall reduced control over release qual-ity. As a result, a single wet well is not conducive to blending water fromdifferent elevations. However, with a system of two or more wet wells, oneport in each wet well can be opened to blend waters from different elevationsto meet downstream water quality objectives. Multilevel outlets can also beused to pass density flows through reservoirs, but port capacities are gen-erally limited to those capacities used in normal operation. Also, larger

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flows can be passed by combining a flood control outlet or spillway with themultilevel intake structure. However, using spillway releases and hydropowerreleases may not result in a uniform downstream quality. Such waters may notreadily mix because of density differences. The flood control outlet is com-monly a large-capacitybottom outlet. Flood control operation, however, gen-erally results in temporary loss of control over release quality unlessnonflood flows also are discharged through the bottom outlet or over thespillway.

e. Water Budget.(1) The water balance in reservoirs is the result of the income of andlosses from the reservoir and can have a significant effect on reservoir and

release water quality on an annual, seasonal, daily, and even hourly basis.The income may consist of precipitation on the water surface, tributaryinflow, watershed runoff, point source discharges, and ground water. Waterlosses occur through evaporation from the water surface, evapotranspirationbyaquatic plants, reservoir withdrawals, leakage, and ground-water recharge orseepage. The change in water storage is a function of the difference betweenincome and loss.

2) The total water budget varies from wet year to dry, season to sea-son, and day to day. Any assessment of water qualtty requires a clear under-standing of the projects water budget and the variability of that budget withtime. Factors such as chemical concentrations and stratification, turbidity,productivity, thermal regime, and sediment transport are strongly influencedby a projects water budget.

f. water Properties. Water has several unique physical properties thatmust be considered in water quality assessment. These properties include den-sity, specific heat, viscosity, and surface tension.

(1) Density. Water has its maximum density, 1 gram per milliliter, near4 C (Figure 2-6) and is a nonlinear function of temperature. Water becomesless dense or buoyant as the temperature either increases or decreases from4 c. Ice floats on water, and warmer water floats on cold water because ofthese density differences. Further, the temperature-densityrelation is non-linear; the density difference between 20 and 21 C is approximately equal tothe density difference between 5 and 10 C. Density is also significantlyinfluenced by TDS and SS concentrations. Normally, as TDS and SS concentra-tions increa~e, so does density. Density differenceservoir mixing processes, as well as water quality.

(2) Specific heat. The specific heat of water :kilogram C, which is four times the specific heat ofgains or loses heat more slowly than air. Therefore,air temperatures generally elicit much smaller changesThe large reservoir water mass and high specific heat

influence internal res-

s 1.0 kilocalorie perair. As a result, waterlarge changes in dailyin water temperature.of the water

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0-gg6tli I i i i\II I I I l\0.995-5 0 5 10 15 20 25 30TEMPERATURE, C

Figure 2-6. Water density as a function of temperaturecause temperatures in reservoirs to increase more slowly in the ~ring anddecrease more slowly in the fall than stream or air temperatures.

(3) viscosity. Viscosity is the internal fluid resistance, caused bymolecular attraction, that makes a fluid resistant to flow and is a functionof temperature, decreasing as temperature increases. An illustration of thisproperty is that particulate matter, suspended in the water (i.e., algae,detritus, sediment), will settle faster as temperature increases, sinceviscosity is lower at higher temperatures.4) Surface tension. Surface tension at the air/water interface iscaused by unbalanced molecular attractions that exert an inward adhesion tothe liquid phase (Ref. 110). Surface tension decreases with increasing tem-perature. Also, surface tension can maintain the concentration of debris on

the surface and form a unique microhabitat for microorganisms. Organic com-pounds, either naturally produced dissolved organic carbon (DOC) or organicpollutants such as oil, markedly reduce surface tension.g* Thermal regime. The annual temperature distribution represents oneof the most important limnologicalprocesses occurring within a reservoir.Thermal variation in a reservoir results in temperature-induceddensity strat-ification, and an understanding of the thermal regime is essential to waterquality assessment. A brief discussion of the thermal regime of a reservoir

in the temperate climate is presented in the following paragraphs.(1) Spring thermal regime. As the ice cover deteriorates in the spring,

the surface water, which is near 0 C, begins to warm and approach the temper-ature of the bottom water. Since the density of the surface water increases

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as it approaches 4 C, this surface water sinks and mixes with the water belowit. During this period, there is relatively little thermally induced resis-tance to mixing because of the small density differences, and the reservoirbecomes uniform near 4 C. This period of uniform temperature is referred toas spring turnover. The extent of this period is primarily dependent on in-flow density, wind mixing, and solar insolation. Solar insolation warms thesurface water and thereby establishes a density gradient between the surfaceand underlying water. However, wind energy introduced across the water sur-face stirs the water column and distributes this heat into the water column,resulting in an increase in the temperature of the entire water column to orabove 4 C. As solar insolation intensifies, wind energy no longer can over-come the density gradient between the surface and bottom and completely mixthe water column. As a result, a temperature gradient is established in thewater column, which is called thermal stratification.

(2) Summer thermal regime. During the summer, solar insolation has itshighest intensity and the reservoir becomes stratified into three zones(Figure 2-2).

(a) Epiltiion or mixed layer. This upper zone represents the lessdense, warmer water in the reservoir. It is fairly turbulent since its thick-ness i? determined by the turbulent kinetic energy (TKE) inputs (wind, convec-tion, etc.), and a relatively uniform temperature distribution throughout thiszone is maintained.

(b) Metalimnion. The metalimnion is the middle zone that represents thetransition from warm surface water to cooler bottom water. There is a dis-tinct temperature gradient through the metalimnion. The metalimnion in somereferences is called the thermocline. The thermocline, however, representsthe plane or surface of maxtium rate of change of temperature in themetalimnione(c) Hypolimnion. The hypolimnion is the bottom zone of colder water

that is relatively quiescent in lakes. Bottom withdrawal or fluctuating waterlevels in resenoirs, however, may significantly increase hypolimnetic mixing.3) Fall thermal regime. As solar insolation decreases during autumn

and the air and inflow temperatures cool, reservoir heat losses exceed heatinputs, and water surface temperatures decrease (Figure 2-7). This results inthe surface water becoming denser and mixing with deeper water through windand convection currents, and a reduction of the density difference between themixed layer and hypolimnion. This situation results in a deepening of themixed layer and erosion of the metalimnion. As fall cooling progresses, thewater column eventually reaches a uniform temperature. This period of uniformtemperature in the water column is called fall turnover.(4) Winter thermal regime. Thermal uniformity of the water column will

continue unless the surface water freezes. Ice formation prevents windmixing, and inverse stratification may form under the ice. The bottom wate-

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15~ ,....l.........-

I TEMPERATURE,OC lbr . . . .. .. 0.,. ... ... --- 0 .-. . . - .

JAN APR JUL OCTJANAPRJUL OCT JANAPRJUL OCTJANAPRJUL OCTJANAPR JUL OCTJAN1 8 1 9 1970 1971 1972

Figure 2-7. Recurring annual stratification pattern fortemperate reservoir

or deeper strata may stay near maximum density at 4 C, but the surface watersbecome colder and less dense and offer resistance to mixing, so an inversestratification occurs with 0 C water at the surface and 4 C water near thebottom (Figure 2-7).5) Exceptions. The above discussion of themal regimes in reservoirs(paras 2-7g(l)-(4)) is generally applicable, but, as in most phenomena, thereare exceptions. One exception is that, although spring and fall turnover usu-ally produce uniform temperature in the vertical, the influence of dissolvedand/or suspended constituents can result in the existence of a chemicallyinduced density gradient, particularly in deeper reservoirs. Another excep-

tion is that certain inflow and withdrawal conditions, such as bottom with-drawals that deplete the hypolimnion, can greatly alter the density gradientwithin the pool. As a result, specific characteristics of each reservoir mustbe considered in any water quality assessment.

h. Other Stratification. Density stratification due to a temperaturegradient is the most common type of stratification, but other factors may alsoproduce density differences that result in reservoir stratification. If den-sity differences prevent mixing with the overlying water, the resulting con-dition is called a meromictic or incompletely mixed system. In meromicticreservoirs, the bottom waters are isolated by a monimolimnion, which is simi-lar to the metalimnion. Density differences may be due to physical, chemical,or biological factors.(1) Physical. High suspended sediment concentrations may increase fluiddensities and provide resistance to mixing. Although suspended solids settlerapidly, fine colloidal particles transported into a reservoir during majorstorm events can prevent the bottom waters from mixing with the overlying wa-ter column until settling, dilution, and entrainment eliminate this condition.

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As a consequence, a difference in density between the bottom and overlyingwater can occur for a period after a major storm event.(2) Chemical. High TDS or salinity concentrations can increase water

density and prevent complete mixing of the system. The gradient between theupper mixed layer and lower dense chemical layer is a chemocline.

3) Biological. Decomposition of sediments or sedimenting organic mat-ter can result in salt accumulation that increases the density of the bottomwaters and prevents mixing. This condition, called biogenic meromixis, mayoccur during the initial filling and transition period of a reservoir whendecomposition of flooded soils and vegetation is intense. However, this typeof stratification generally decreases through time,

i. Inflow Mixing Processes. When tributary inflow enters a reservoir,it displaces the reservoir water. If there is no density difference betweenthe inflow and reservoir waters, the inflow will mix with the reservoir wateras the inflow parcel of water moves toward the dam.. However, if there aredensity differences between the inflow and reservoir waters, the inflow movesas a density current in the form of overflows, interflows, or underflows (Fig-ure 2-8). Knapp (Ref. 85) provides an excellent qualitative discussion ofinflow mixing, while Ford and Johnson (Ref. 12) discuss reservoir density cur-rents and inflow processes.

jw Internal Mixing. Internal mixing is the term used to describe mixingwithin a reservoir from such factors as wind, Langmuir circulation, convec-tion, Kelvin-Helmholtz instabilities,and outflow (Figure 2-9). AdditionalI PLUNGE POINT

Figure 2-8. Density inflows to reservoirs (afterRef. 12)

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,. ..,..,..

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topical information is available from the following sources: dynamics oflarge lakes (Refs. 55 and 89 and Item h); mixing dynamics (Refs. 11, 66, 73,and 81); turbulence (Ref. 103); inflow dynamics (Ref. 12); and outflowdynamics and selective withdrawal (Ref. 80). An excellent reference on theinfluence of density stratification on mixing and flow is the Proceedin s ofthe Second International Symposium on Stratified Flow (Ref. 57~

(1) Wind mixing. In many lakes and reservoirs, wind is a major energysource for mixing. Mixing results from the interaction and cumulative effectsof wind-induced shear at the air/water interface (e.g., currents, surfacewaves, internal waves, seiches, and entrainment). Wind is highly variable,with seasonal, synoptic, and diel (24-hour)cycles. Synoptic cycles corre-spond to the passage of major weather systems or fronts and have a period of5 to 7 days. Wind is an important factor influencing the depth of the mixedlayer. Langmuir circulations are wind-induced surface currents that move asvertical helices. Wind energy is converted into turbulence by many differentprocesses, including the direct production of turbulence. This surface turbu-lence is transported downward and mixes water until the density gradient orthermal resistance to mixing dissipates the energy, resulting in the mixedlayer depth.

2) Convection. Convective mixing results from density instabilitiesdue to cooling of surface waters. As the surface water cools it becomes moredense and settles, mixing with underlying strata. Penetrative convectivecooling during the fall can be an important factor in deepening of the mixedlayer and erosion of the metalimnion (Figure 2-10).HATURALCONVECTION

1[NOHPENETRATIVE

T

7

IPENETRATIVE

Figure 2-10. Influence of penetrative convectivemixing on deepening the mixed layer(3) Kelvin-Helmholtz instabilities. Internal and surface waves andseiches transport momentum but contribute little mixing unless energy is dis-sipated through shear or friction. When internal waves become unstable andbreak, the process is referred to as a Kelvin-Helmholtz instability, andmixing occurs at the interface. Since reservoir operation can result in

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fluctuating water levels and unsteady flow, significant mixing can occur atvarious interfaces in the reservoir such as the sediment/hypolimnion inter-face, meta/hypolimnion, and epi/metalimnion interface.

4) Outflow mixing. When water is released from a reservoir, potentialenergy is converted into kinetic energy. Mixing is a result of this conver-sion of energy, although restricted to the zone of outflow, and is propor-tional to the third power of the discharge. The outflow zone is a function ofthe stratification regime and the hydraulic outlet geometry and operation(Refs. 7, 22). Hypolimnetic or bottom withdrawal can increase mixing in thehypolimnion and alter the stratification profile in the pool. Hydroelectricpower generation can significantly increase mixing in the pool.

k. Pumped Storage. Pumped-storage hydroelectric power operations canincrease mixing both through outflow mixing and through mixing and entrainmentof the pumpback jet into the reservoir. Depending on the elevation of theinlets, the pumpback jet may move as a density flow entraining the surroundingwater until it reaches a level with comparable density (Figure 2-11). Thismixing can result in the vertical movement of hypolimnetic constituents intothe upper waters.

1. Sediment Dynamics. Sedf.mentdeposition patterns and sediment qual-ity, ho~ver, markedly influence reservoir water quality. Sediment charac-terization, yield, transport mechanics, and other aspects of sedimentationengineering (Ref. 106) are important considerations.m. Deposition Patterns. Gravels, sands, and other coarse sediments are

deposited in the reservoir delta and do not influence water quality. Thereservoir delta is defined as the deposition zone between the maximum normalflood pool and the normal conservation pool. Suspended sediment transportedinto the reservoir typically ranges from coarse silts and particulate organicmatter to fine clays and colloidal organic matter. As turbulence and rivervelocities decrease in the reservoir headwater, the sediment-carrying capacityof the river decreases and sediments are deposited. Since the river and itsconstituent load generally follow the old thalweg through the reservoir, sedi-ment deposition initially is greatest in the old channel. Sedimentation anddeposition rates are highest in the headwater and decrease exponentially downthe reservoir with plug flow characteristics. This results in a longitudinalsorting of particulate matter by particle size. The coarse silts and organic_particles settle in the upper portion of the reservoir; fine silts, coarseclays, and finer organic particles settle next, with the fine clays and col-loidal material settling very slowly. Finally, sediment deposition patternsare extremely complex and reflect the interaction of inflow patterns and stor-age patterns as well as physical, chemical, biological, and seasonal factorsthat affect the water and the watershed. An example of a typical distributionof deposited particle size in a reservoir is:

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a. Initial conditions,typical density stratification

b. Buoyant jetting inflow

c Shortly after inflow ceases

d. Long time after inflow ceases

Densitycurrent

3.inal *S*,density ;profile ~ Initial: density; profileIFigure 2-11. Important hydrodynamic features of pumped-storagereservoirs subject to jetting inflows (afterRef. 21)

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Percent of Deposited SedimentParticle Size Inlet Mid-Reservoir OutletSand 5


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but does stratify. The loading and sedimentation ofthis transition area and, during stratification, the

EM 1110-2-120130 Jun 87

organic matter is high inhypolimnetic DO to sat-isfy this demand can be depleted. If anoxic conditions-develop,they gener-ally do so in this area of the reservoir and progressively move toward the damduring the stratificationperiod. The SOD is relatively independent of DOwhen DO concentrations in the water column are greater than 3 to 4 milligramsper liter but becomes limited by the rate of oxygen supply to the sediments.(b) Water column demand. A characteristic of many reservoirs is a meta-limnetic minimum in DO concentrationsor negative heterograde oxygen curve(Figure 2-12). Density interflows not only transport oxygen-demandingmate-rial into the metalimnion but can also entrain reduced chemicals from the

....DIS~VED OXWENy~~ ~

II 1 I

Figure 2-12. Characteristicmetalimnetic DO minimumupstream anoxic area and create additional oxygen demand. Organic matter andorganisms from the mixed layer settle at slower rates in the metalimnion be-cause of Increased viscosity due to lower temperatures. Since this labileorganic matter remains in the metalimnion for a longer time period, decomposi-tion occurs over a longer time, exerting a high oxygen demand. Metalimneticoxygen depletion is an important process in deep reservoirs. A hypolimneticoxygen demand generally starts at the sediment/water interface unless under-flow contribute organic matter that exerts a significant oxygen demand. Inaddition to metalimnetic DO depletion, hypolimnetic DO depletion also isimportant in shallow, stratified reservoirs since there is a smaller hypo-limnetic volume of oxygen to satisfy oxygen demands than in deep reservoirs.

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(c) Dissolved oxygen distribution. Two basic types of vertical oxygendistribution may occur in the water column: an orthograde and clfnogradeoxygen distribution (Figure 2-13). In the orthograde distribution, oxygenconcentration is a function primarily of temperature, since oxygen consumptionis limited. The clinograde oxygen profile is more representative of Corpsreservoirs where the hypolimnetic oxygen concentration progressively decreasesduring stratification (Figure 2-13) and can occur during both summer andwinter stratification periods.

I1/0

//

CLINOGRADE

Figure 2-13. Orthograde and clinograde vertical DO distributions(2) Inorganic carbon. Inorganic carbon represents the basic building

block for the production of organic matter by plants. Inorganic carbon canalso regulate the pH and buffering capacity or alkalinity of aquatic systems.Inorganic carbon exists in a dynamic equilibrium in three major forms: carbondioxide (C02), bicarbonate ions (HCO~), and carbonate ions (CO;). Carbondioxide is readily soluble in water and some C02 remains in a gaseous form,but the majority of the C02 forms carbonic acid which dissociates rapidly intoHCO~ and CO; ions. This dissociation results in a weakly alkaline system(i.e., pH - 7.1 or 7.2). There is an inverse relation between pH and C02.When aquatic plants (plankton or macrophytes) remove C02 from the water toform organic matter through photosynthesis, the pH increases. The extent ofthis pH change provides an indication of the buffering capacity of the system.Weakly buffered systems with low alkalinities (i.e.,


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liter) experience larger shifts in pH than well-buffered>1,000 microequivalents per liter).

EM 1110-2-120130 Jun 87

systems (i.e.,

3) Nitrogen. Nitrogen is important in the formulation of plant andanimal protein. Nitrogen, sf.milarto carbon, also has a gaseous form. Manyspecies of blue-green algae can use or fix elemental or gaseous N2 as a nitro-gen source. The most common forms of nitrogen in aquatic systems are ammonia(NH3-N), nitrite (N02-N), and nitrate (N03-N). All three forms are trans-ported in water in a dissolved phase. Ammonia results primarily from thedecomposition of organic matter. Nitrite is primarily an intermediate com-pound in the oxidation or vitrification of ammonia to nitrate, while nitrateis the stable oxidation state of nitrogen and represents the other primaryinorganic nitrogen form besides NH3 used by aquatic plants.

(4) Phosphorus. Phosphorus is used by both plants and animals to formenzymes and vitamins and to store energy in organic matter. Phosphorus hasreceived considerable attention as the nutrient controlling algal productionand densities and associated water quality problems. The reasons for thisemphasis are: phosphorus tends to lf.mitplant growth more than the othermajor nutrients (see Table 2-3); phosphorus does not have a gaseous phase andultimately originates from the weathering of rocks; removal of phosphorus frompoint ~urces can reduce the growth of aquatic plants; and the technology forremoving phosphorus is more advanced and less expensive than nitrogen removal.

TABLE 2-3Nutrient Demand:SuppIy Ratios During Nonproductive and

Productive Seasonsl

Demand:Supply (range)Element Late Winter MidsummerPhosphorus 80,000 800,000Nitrogen 30,000 300,000Carbon 5,000 6,000Iron, silicon Generally low, but variableAll other elements Less than 1,000

1After Item SS, Appendix B.

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Phosphorus is generally expressed in terms of the chemical procedures used formeasurement: total phosphorus, particulate phosphorus, dissolved or filter-able phosphorus, and soluble reactive phosphorus (SRP). Phosphorus is a veryreactive element; it reacts with many cations such as iron and calcium and isreadily sorbed on particulate matter such as clays, carbonates, and inorganiccolloids. Since phosphorus exists in a particulate phase, sedimentation rep-resents a continuous loss from the water column to the sediment. Sedimentphosphorus, then, may exhibit longitudinal gradients in reservoirs similar tosediment silt/clay gradients. Phosphorus contributions from sediment underanoxic conditions and macrophyte decomposition are considered internal phos-phorus sources or loads, are in a chemical form available for plankton uptakeand use, and can represent a major portion of the phosphorus budget.

5) Silica. Silica is an essential component of diatom frustules orcell walls. Silica uptake by diatoms can markedly reduce silica concentra-tions in the epilimnion and initiate a seasonal succession of diatom species(Ref. 110). When silica concentrations decrease below 0.5 milligram perliter, diatoms generally are no longer competitive with other planktonspecies.

6) Other nutrients. Iron, manganese, and sulfur concentrations gen-erally are adequate to satisfy plant nutrient requirements. Oxidized iron(III) and manganese (IV) are quite insoluble in water and occur in low concen-trations under aerobic conditions. Under aerobic conditions, sulfur usuallyis present as sulfate.

b. Gas Exchange.(1) Gas exchange across the air/water interface is a function of atmos-pheric pressure, temperature, concentration gradients, and turbulence. The

volubility of most gases in water is directly proportional to the partialpressure in the gaseous phase (Henrys Law) and decreases in a nonlinear man-ner with increasing temperature and altitude (i.e., decreasing atmosphericpressure). Gas transfer is directly proportional to the concentration gra-dient and turbulence at the air/water interface; however, molecular diffusionis an insignificant mechanism for gas exchange.

(2) Gas exchange across the air/water interface occurs for several gasesother than oxygen. Nitrogen, both as elemental nitrogen and ammonia-N, andcarbon dioxide also diffuse in and out of the water across this interface.Methane and hydrogen sulfide are two gases that are occasionally produced inthe reservoir and may be released across the air/water interface. Water alsocan, on occasion, become supersaturatedwith gases. Release of supersaturatedgaseous nitrogen, methane, and hydrogen sulfide in reservoir releases can be amajor water quality concern in the tailwater.

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c. Anaerobic (Anoxic) Conditions.(1) General. When DO concentrations in the hypolimnion are reduced toapproximately 2 to 3 milligrams per liter, the oxygen regime at the sediment/

water interface is generally considered anoxic, and anaerobic processes beginto occur in the sediment interstitial water. Nitrate reduction to ammoniumand/or N20 or N2 (denitrification)is considered to be the first phase ofanaerobic processes and places the system in a slight y reduced electrochemi-cal state. Ammonium-nitrogen begins to accumulate in the hypolimnetic water.The presence of nitrate prevents the production of additional reduced formssuch as manganese (II), iron (11), or sulfide species. Denitrification proba-bly serves as the main mechanism for removing nitrate from the hypolimnion.Following the reduction or denitrification of nitrate, manganese species arereduced from insoluble forms (e.g., Mn (IV)) to soluble manganous forms (e.g.,Mn (11)), which diffuse into the overlying water column. Nitrate reduction isan important step in anaerobic processes since the presence of nitrate in thewater column will inhibit manganese reduction. As the electrochemical poten-tial of the system becomes further reduced, iron is reduced from the insolubleferric (III) form to the soluble ferrous (II) form, and begins to diffuse intothe overlying water column. Phosphorus, in many instances, is also trans-ported in a complexed form with insoluble ferric (III) species so the reduc-tion and solubilization of iron also result in the release and solubilizationof phosphorus into the water column. The sediments may serve as a major phos-phorus source during anoxic periods and a phosphorus sink during aerobicperiods (Figure 2-14). During this period of anaerobiosis, microorganismsalso are decomposing organic matter into lower molecular weight acids andalcohols such as acetic, fulvic, humic, and citric acids and methanol. Thesecompounds may also serve as trihalomethaneprecursors (low-molecularweightorganic compounds in water; i.e., methane, formate acetate) which, when sub-ject to chlorination during water treatment, form trihalomethanes,or ms(carcinogens). As the system becomes further reduced, sulfate is reduced tosulfide, which begins to appear in the water column. Sulfide will readilycombine with soluble reduced iron (11), however, to form insoluble ferroussulfide, which precipitates out of solution. If the sulfate is reduced tosulfide and the electrochemical potential is strongly reducing, methane for-mation from the reduced organic acids and alcohols may occur. Consequently,water samples from anoxic depths will exhibit chemical characteristics.

2) Spatial variability. Anaerobic processes are generally initiated inthe upstream portion of the hypolimnion where organic loading from the inflowis relatively high and the volume of the hypolimnion is minimal, so oxygendepletion occurs rapidly. Anaerobic conditions are generally initiated at thesediment/water interface and gradually diffuse into the overlying water columnand downstream toward the darn. Anoxic conditions may also develop in a deeppocket near the dam due to decomposition of autochthonous organic matter set-tling to the sediment. This anoxic pocket, in addition to expanding verti-cally into the water column, may also move upstream and eventually meet theanoxic zone moving downstream. (Additional information is provided inItem gg.)

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EXTERNAL

EXTERNAL

LOADING

LOADING

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WINTER ANDSPRING

11.... .. ...- - ... .EARLYSUMMER RELEASE FROM ;:;:SEDIMENTS

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3) Vertical variability. Anoxic conditions are generally associatedwith the hypolimnion, but anoxic conditions may occur in the metalimnion. Themetalimnion may become anoxic due to microbial respiration and decompositionof plankton settling into the metalimnion, microbial metabolism of organicmatter entering as an interflow, or through entrainment of anoxic hypolimneticwater from the upper portion of the reservoir.

(d) Initial filling. Reservoirs undergo dynamic chemical and biologicalchanges during the first 6 to 10 years following impoundment. This periodfollowing initial inundation has been termed the trophic upsurge period and isgenerally characterized by increased productivity, although productivity ini-tially may decrease due to high turbidity. The increased productivity isattributed to the rapid decomposition and leaching of organic matter andnutrients from the inundated soil, humus, litter, and herbaceous and woodyvegetation. Decomposition and nutrient leaching rates are a function of manyvariables such as temperature, chemical composition, and cellulose content butare directly proportional to the particle surface area to volume ratio.Pieces of grass, humus, etc., have a larger surface area to volume ratio thanlimbs and branches. In addition, vegetation high in cellulose, such as stand-ing timber, generally degrades very slowly while grasses and herbaceous vege-tation decompose rapidly. Decomposition of this organic material exerts asignificant oxygen demand. If the reservoir stratifies, the hypolimniongenerally is anoxic for the first several years until this demand is satis-fied. The hypolimnetic and release water, then, may contain high concentra-tions of reduced constituents such as Mn (II), Fe (II), H2S, and possiblymethane. The decline in oxygen demand through time (i.e., 2 to 4 years) isroughly exponential. Decomposition of this organic matter results in highnutrient concentrations,which may stimulate algal production. Benthic pro-ductivity also is high during this period since detritus and particulateorganic carbon (POC) concentrations are readily available for consumption.Algal and benthic productivity typically result in good fish production duringthis trophic upsurge period.2-9. Biological Characteristics and Processes.

a. Meromixis. Decomposition of organic matter in sediments or sedi-menting organic matter can increase salinity concentrations,which increasesthe density of the water and prevents mixing. This condition, called mero-mixis, may occur during the initial filling and transition period of a reser-voir when decomposition of flooded soils and vegetation is intense. This typeof stratification generally decreases through time.

b. Microbiological. The microorganisms associated with reservoirs maybe categorized as pathogenic (to man and other organisms) or nonpathogenic.Pathogenic microorganisms, including viruses, are of concern from a humanhealth standpoint in that they may limit recreational use. Nonpathogenicmicroorganismsorganic matterMicroorganisms

are important in that they often serve as decomposes ofand are a major source of carbon and energy for a reservoir.generally inhabit all zones of the reservoir (riverine,

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transition, and lacustrine) as well as all layers (hypolimnion,metalimnion,and epilimnion). Seasonally high concentrations of bacteria will occur duringthe warmer months but can be diluted by high discharges. Anaerobic conditionsenhance growth of certain bacteria while aeration facilitates the use of bac-terial food sources. Microorganisms, bacteria in particular, are responsiblefor mobilization of other contaminants from sedf.ments.

c. Photosynthesis. Oxygen is a by-product of aquatic plant photosyn-thesis, which represents a major source of oxygen for aquatic ecosystems dur-ing the growing season. Oxygen volubility is less during the period of higherwater temperatures, and diffusion may be less because wind speeds are usuallylower during the summer than the spring or fall. Biological activity andoxygen demand typically are high during stratification, so photosynthesismayrepresent the major source of oxygen during this period. Oxygen supersatura-tion in the euphotic zone can occur during periods of high photosynthesis.

d. Phytoplankton and Prf.maryProductivity. Phytoplankton influence DOand suspended solids concentrations, transparency, taste and odor, aesthetics,and other factors that affect many reservoir uses and water quality objec-tives. Phytoplankton are the primary source of organic matter production andform the base of the autochthonous (i.e., organic matter produced in the sys-tem) fo~d web in many reservoirs since fluctuating water levels may lf.mitmacrophyte and periphyton production. Phytoplankton species are classifiedaccording to standard taxonomic nomenclature but are usually described by gen-eral descriptive names such as diatoms, greens, blue-greens, or cryptomonadalgae. Phytoplankton species identification and biomass estimates representstatic measures of the plankton assemblage, while plankton succession and pri-mary production are dynamic or the-varying measures of the plankton assem-blage. Chlorophyll ~ represents a common variable used to esttiate planktonbiomass while light-dark bottle oxygen measurements or C14 uptake are used toestimate primary production. Phytoplankton species in reservoirs are identi-cal to those found in lakes. However, since growth of the phytoplankton iscontrolled by the physiochemical conditions in reservoirs, the planktonresponse or spatial variability may vary by reservoir.

e. Temporal Variability. Seasonal succession of phytoplankton speciesis a natural occurrence in lakes and reservoirs (Figure 2-15). The springassemblage is usually dominated by diatoms and cryptomonads. Silica depletionin the photic zone and increased settling as viscosity decreases because ofincreased temperatures usually result in green algae succeeding the diatoms.Decreases in nitrogen or a decreased competitive advantage for carbon athigher pH may result in blue-green algae succeeding the green algae duringsummer and early fall. Diatoms generally return in the fall but blue-greens,greens, or diatoms may cause algae blooms following fall turnover when hypo-ltietic nutrients are mf.xedthroughout the water column. The general patternof seasonal succession of phytoplankton is fairly constant from year to year.However, hydrologic variability, such as increased mixing and delay in theonset of stratification during wet spring periods, can maintain diatoms longerin the spring and shift or modify the successional pattern in reservoirs.

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BLUE GREENWvzaQ DIATOMSz3maW~F~

1 t 1 1 1JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC JAN

Figure 2-15. Seasonal patterns of phytoplankton successionf. Macrophytes. Macrophytes or large aquatic plants can be representedby four types of plants: emergent, floating leaved, submerged, or free-floating. Macrophytes generally inhabit the littoral zone or interface zone

between the waters edge and the open-water expanse of the reservoir (Fig-ure 2-16). The maximum depth at which attached macrophytes occur is10 meters, but light penetration generally limits macrophytes to shallowerdepths. Fluctuating water levels markedly reduce reservoir macrophy~es.bydesiccating and/or freezing the species, although some species are stimulatedby fluctuatingwater levels. Rooted macrophyte species are capable of absorb-ing nutrients from either the sediment or water column. Since nutrient con-centrations are usually greater in the sediment than the water column,sediments represent a major source of nutrients for macrophytes. Nutrientsremoved from the sediments can be released into the overlying water column asmacrophyte tissue decays and can contribute to the internal loading ofnutrients in reservoirs. Macrophytes, particularly floating leaved and free-floating species, may compete with phytoplankton for available light. Free-floating species also compete with algae for nutrients. Both free-floatingspecies and algae may limit light so that submersed macrophyte species cannotgrow.

g* Periphyton. Periphyton algae grow attached to a substrate such asrocks, sand, macrophytes, or standing timber. Periphyton attached to standingtimber in the headwater of reservoirs may serve two functions. First, peri-phyton may remove nutrients from the inflowing tributary and reduce the nu-trients available for reservoir phytoplankton. Second, the periphyton serveas a food resource for the benthos and, directly or indirectly, for fishspecies.

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h. Secondary and Tertiary Productivity. Secondary and tertiary produc-tivity refer to consumption in an ecological food chain (Figure 2-17). Plank-ton grazers such as zooplankton, benthos, and fish are considered primaryconsumers, or the first level above the plant producers. Since primary pro-ductivity represents the first level or base of productivity, primary con-sumers represent the second level of productivity, or secondary production(Figure 2-17). Zooplankton, benthic, and fish species that consume thegrazers represent tertiary production and secondary consumers (Figure 2-17).Secondary and tertiary production may not directly influence water quality butcan have a significant indirect role in reservoir water quality. Phytoplank-ton, macrophyte, and periphyton consumers or grazers can reduce the abundanceof these species and alter succession patterns. The white amur or Asian carphas been used effectively to control macrophytes through consumption. Somephytoplankton species are consumed and assimilated more readily and are pref-erentially selected by consumers. Single-celled diatom and green algae spe-cies are readily consumed by zooplankton while filamentous blue-green algaeare avoided by zooplankters. Larger zooplankton can consume larger planktonspecies, but these larger zooplankton species are also selected by planktivor-OUS fish. Altering the fish population can result in a change in the

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Reservoir Water Quality Analysis